Communications systems of the above type developed from the well known wired broadcasting systems in which television and sound signals are distributed to subscribers. Known wired broadcasting systems have generally followed one of two basic design approaches, that is the braodband approach or the star network approach.
In broadband systems all signals which are to be available to subscribers are broadcast simultaneously on a coaxial cable and signal selection is effected by tuning of the subscribers receiver. Thus the broadcast signals are frequency division multiplexed. This approach has the advantage that the cable network is relatively simple but the disadvantage that the number of different signals which can be transmitted is limited by the available bandwidth. Furthermore there is now a demand not only for an increased number of different signals but also for two way communication, i.e. the capacity for subscribers to transmit data to the system, not merely to receive signals from it. Broadband systems which can provide the subscribers with the facility of transmitting signals and which can identify the subscriber source of transmitted signals are complex, particularly when the number of subscribers is large. Generally some form of time division multiplexing has to be employed to manage the subscriber transmitted signals.
In star network systems each subscriber is connected by his own cable to a signal distribution centre commonly referred to as an exchange. At the exchange are located selectors controlled by the subscribers for connecting the subscriber cables to desired signal sources all operating at the same signal frequency. There is thus no theoretical limit on the number of different signals which can be broadcast to the subscribers even though each subscriber has only one cable extending to his premises. Furthermore the source of data signals transmitted by a subscriber is identified by the cable on which the data signals appear. Thus star networks are inherently more suitable for use in high signal capacity two way systems. The basic cable network is however more complex as each subscriber must be linked directly by his own cable to the exchange. In practice in all but small systems a series of exchanges are provided on a trunk cable network, each exchange serving a localized group of subscribers.
It is now frequently necessary to be able to provide a single subscriber with more than one outlet on his premises, and it is generally thought that a capacity to serve three outlets per subscriber is required when the cable for a new system is to be installed. In a broadband system this does not raise any real problems, providing that the problems inherent in high capacity two way braodband systems can be overcome. In simple star network systems however it would be necessary to provide each subscriber with three cables in case he eventually decided to use three outlets. The expense involved would be prohibitive. It will be appreciated that if subscribers were initially provided with only one cable it would not be economically feasible to subsequently install further cables as subscriber demand required.
Furthermore, in large systems comprising many exchanges the volume of data to be transmitted from the exchanges to the head end is considerable. Although the source of data signals reaching the exchanges from subscriber stations is identified by the cable on which the signals appear it is nevertheless necessary to "label" the source of the signals when the data is transferred to the head end. If this is achieved by associating an address code with each time of data to be transmitted to the head end the data handling capacity of the system is strained further.
It is an object of the present invention to provide a communications system in which the source of subscriber generated signals can be readily determined.